1. Field of the Invention
The present invention relates to a developing device using a two-component developer and an image forming apparatus having the developing device.
2. Description of the Related Art
In recent years, a two-component developer (hereinafter, simply “developer”) having excellent toner charging stability is widely used in electrophotographic type image forming apparatuses that cope with full colors and high image quality.
This developer is composed of a toner and a carrier. The toners and carriers are agitated in a developer tank of a developing device and they are rubbed so that the toner that is suitably charged by the rubbing can be obtained.
In the developing device, the charged toner is supplied to a surface of a developing roller, and is transferred from the developing roller to an electrostatic latent image formed on a surface of a photosensitive drum by an electrostatic attractive force. As a result, a toner image is formed on the photosensitive drum based on the electrostatic latent image.
Recently, there is a demand for a high-speed, miniaturized image forming apparatuses. Accordingly, it is necessary to charge a developer quickly and sufficiently, and convey the developer quickly.
For this reason, as a conventional technique 1, a circulation type developing device is proposed (for example, see Japanese Patent Application Laid-Open No. 2001-255723). This developing device has first and second developer conveying paths, first and second connecting paths and first and second auger screws. The first and second developer conveying paths are partitioned by a partition wall provided in a developer tank. The first and second connecting paths connect the first developer conveying path and the second developer conveying path at both ends. The first and second auger screws are arranged on the first and second developer conveying paths and convey the developer to opposite directions. The developing device circulates the developer in the first and second developer conveying paths, and draws the developer in the second developer conveying path to supply it to the photosensitive drum. Thereafter, the developing device returns the developer from the photosensitive drum to the second developer conveying path.
In a case of the image forming apparatus of the conventional technique 1, when an image whose image density is locally high is printed and thus a toner in the developer on a developing roller is consumed locally and notably, the toner density in the developer on the developing roller is locally low.
Such a developer with low toner density is returned into the second developer conveying path. When the developer with low toner density is supplied again to the developing roller before being mixed with another developer sufficiently, an amount of toner to be supplied to the photosensitive drum is locally reduced. As a result, unevenness of image density (a phenomenon as so-called “development memory”) occurs. This phenomenon such that the image density of a printed image obtained by printing an image with locally high density is locally low.
In order to solve this problem, a conventional technique 2 proposes the following developing device. This developing device has first and second developer conveying paths, first and second connecting paths, first and second auger screws, a third developer conveying path, and a third auger screw. The first and second developer conveying paths are partitioned by a partition wall provided into a developer tank. The first and second connecting paths connect the first developer conveying path and the second developer conveying path at both ends. The first and second auger screws are arranged in the first and second developer conveying paths and convey a developer to opposite directions. The third developer conveying path is arranged above the first and second developer conveying paths. The third auger screw is arranged in the third developer conveying path. The developing device circulates the developer in the first and second developer conveying paths, and draws the developer in the second developer conveying path so as to supply the developer to a photosensitive drum. Thereafter, the developing device returns the developer to the first developer conveying path via the third developer conveying path, so that occurrence of the unevenness of image density is repressed. There is a conventional technique disclosed in Japanese Patent Application Laid-Open No. 2008-26408.
However, in the developing device of the conventional technique 2, three developer conveying paths and three auger screws are necessary for repressing the occurrence of unevenness of image density, and thus the developing device is enlarged and is complicated.